1. Field of the Invention
This invention relates to methods for sampling and determining the Total Organize Carbon is an aqueous liquid.
2. Background and Prior Art
Total Organic Carbon (TOC) measurements are not new in analytical chemistry. The methods have not been developed to the extent that analyte-specific methods have been because TOC is less glamorous and typically less useful. TOC is becoming more important as regulatory focuses change and limits of detection are lowered. TOC is the method of choice for determining the purity of water in the pharmaceutical industry and for analysis of drinking water. It is increasingly recognized as a preferred method for characterizing discharges from water treatment facilities. In each case, specific target species may be present at below detectable limits but total organics are measurable and give a good measure of overall contamination. This is especially the case when the sources(s) and identities of the carbon are known.
In addition to drinking water safety and pharmaceutical safety, ultrapure water is required in many industries including semiconductor fabrication.
The TOC measurement methods all require the steps of sample collection, sample volume reduction (concentration), sample oxidation and measurement of the common oxidation product (usually CO.sub.2).
Samples are obtained in bulk--the so-called grab sample--from an environmental source, from a pipeline or from a packaged sample such as water for injection. The concentration of analyte may be extremely low and volume reduction is required.
Volume reduction has historically been done by some form of distillation. The exact process is complicated by the presence of purgeable organics (VOC's) is many samples. U.S. Pat. No. 3,859,209 to Jahnsen et al. discloses a method for separating organics from a large volume of solvent and loading onto a GC column by a form of flash evaporation. U.S. Pat. No. 4,180.389 to Paul describes a process whereby the sample is purified by a form of repetitive purge and trap steps.
U.S. Pat. No. 5,401,468 to Parashnick et al. is directed to a method for the determination of total carbon in the exhaust from a diesel engine using a closed loop and a ceramic trap. No provision is made to separate organic from inorganic carbon.
U.S. Pat. No. 5,425,919 to Inone et al. describes a TOC analyzer which uses an absorbing agent containing Ba(OH).sub.2 to separate and quantify purgeable organic carbon in a TOC determination.
U.S. Pat. No. 5,574,230 to Baugh discloses an air sampling tube containing silica gel, Tenax and activated carbon for introduction by thermal desorption into a GC/MS.
The oxidation to CO.sub.2 may be effected in a variety of ways. U.S. Pat. No. 4,619,902 to Bernard discloses an improved wet digestion method using persulfate and a variety of catalysts. U.S. Pat. Nos. 5,292,666 and 5,340,542 to Fabinski et al. disclose methods for determination of TOC and total nitrogen in water using thermal reactors and non-dispersive infrared gas analyzers.
U.S. Pat. No. 5,315,885 to Szinyei describes one method for quantitatively introducing a sample into a pyrolysis furnace. U.S. Pat. No. 5,567,388 to Morita et al. discloses a method for returning purgeables to the "organic side" of a semipermeable membrane after separation of inorganic carbon prior to pyrolysis.
Godec et al., U.S. Pat. No. 5,750,073 discloses a third method for conversion to CO.sub.2 and measurement, viz oxidation using short wavelength UV lights and measurement of total carbon electrochemically based on breakdown products.
The sensitivity of the prior art methods of TOC analysis depends upon the sensitivity of the detection method. There remains a need for a method for concentrating the sample to effect lower detection limits which does not risk loss of purgeable organics and which minimizes handling losses between sampling and detection.